1. Field of the Invention
This invention relates to improvements in fluidic proximity sensors in general and more particularly to a proximity sensor which provides high sensitivity with a zero output signal for various lengths of separation between the sensing nozzle and the emitter assembly.
2. Description of the Prior Art
Proximity sensors have heretofore comprised back pressure devices which provide a pressure output signal which is a function of the sensing nozzle clearance. To increase the sensing range of these devices the amount of air flowing through the sensing nozzle is increased.
One means of increasing the air flow from the sensing nozzle is to increase the bore diameter of the nozzle. An undesirable side effect is introduced, however, since the sensor's sensitivity to small objects is reduced. Furthermore, the output zero point changes and requires compensation.
Another means of increasing the air flow from the sensing nozzle is to increase the nozzle velocity. This is done by raising the supply pressure level. However, the output pressure zero point is raised and therefore requires some type of zero compensation. Continuous bleeding of the output to reduce it to a zero pressure level is possible. This type of zero adjustment is range coupled and is therefore impractical since a change in the zero output level also changes the range or sensitivity of the sensor.
Most proximity sensors have their sensing nozzles remotely mounted from the emitter assembly and have various lengths and diameters of tubing connecting the two. The resistance of the various possible connecting tubes is a function of their length and diameter and affects the back pressure transmitted from the sensing nozzle to the output chamber. This results in various zero level output signal compensation being needed depending upon the length and diameter of the tubing used.